A new optimistic synchronization algorithm for Parallel Discrete Event Simulation (PDES) called Safe BTW is proposed in this paper. This new algorithm eliminates risky event processing in the Time Warp processing stage of the original BTW algorithm and is founded on a concept called "safe causal relation". In our new algorithm, the length of any chained rollback operations is limited to at most one and cascading rollback effects found in the original BTW algorithm can be completely avoided. Performance tests of Time Warp, Breathing Time Bucket, Breathing Time Warp and the new Safe BTW algorithms using PHOLD model show that the new algorithm obtains better speedups as the computation scales up. Its effective event processing time as a percentage of the total event processing time is higher than that of Time Warp and Breathing Time Warp, proving that excessive rollback operations are successfully controlled. And the extra synchronization overhead it introduces is still much lower than that of Breathing Time Bucket itself and is within tolerable and stable level as the computation scales up.